inbound_email: introduce view-based email processing
This introduces a view that takes an email as a file upload, and ingests it into the regular inbound email flow.
To do so, the tests have been re-jigged to use a fixture to inject the email, keeping compatibility with the old code.
The current view-based code is broken, probably because I didn't use the mixins for the CBV in the right order...
TODO:
-
fix the view to actually do something instead of silently returning a 200 code -
making sure that the tests test the function